The present invention relates to optical switch networks and, more particularly, to a strictly nonblocking tree network architecture with low crosstalk and efficient use of space.
Tree networks are reviewed in Andrzej Jajszczyk and H. T. Mouftah, "Tree-type photonic switching networks", IEEE Network, vol. 9 no. 1 pp. 10-16 (1995), which is incorporated by reference for all purposes as if fully set forth herein. FIG. 1 shows the high-level layout of a generic tree network for connecting P input waveguides 12 to Q output waveguides 14. Input waveguides 12 enter a branching region 16, where an array of 1.times.2 splitters connects input waveguides 12 to PQ branching region output waveguides 22. Output waveguides 14 emerge from a combining region 20 where an array of 2.times.1 combiners connects output waveguides 14 to PQ combining region input waveguides 24. Waveguides 22 and 24 are connected in an interconnection region 18 in a manner that allows any input waveguide 12 to be connected to any combination of output waveguides 14.
FIG. 2 shows a classical 4.times.4 tree network architecture, for connecting four input waveguides 12 to four output waveguides 14. The four input waveguides 12 are indexed serially by binary indices 00, 01, 10 and 11. Similarly, the four output waveguides 14 are indexed serially by binary indices 00, 01, 10 and 11.
Input waveguides 12 enter a branching region 16 that includes two branching cascades 30 of 1.times.2 splitters 26. Input waveguides 12 are the input waveguides of the first branching cascade 30. The eight output waveguides of the first branching cascade 30 are indexed, relative to input waveguides 12, in a manner that is referred to herein as "least significant inserted bit order". For each 1.times.2 splitter of the first branching cascade 30, the index of the upper output waveguide is obtained by appending a zero to the index of the input waveguide, and the index of the lower waveguide is obtained by appending a one to the index of the input waveguide. The eight output waveguides of the first branching cascade 30 are the eight input waveguides of the second branching cascade 30, and the sixteen output waveguides of the second branching cascade 30 are indexed relative to the eight input waveguides of the second branching cascade 30 in least significant inserted bit order.
Similarly, output waveguides 14 emerge from a combining region 20 that includes two combining cascades 32 of 2.times.1 combiners 28. Output waveguides 14 are the output waveguides of the second combining cascade 32. The eight input waveguides of the second combining cascade 32 are indexed, relative to output waveguides 14, in least significant inserted bit order. The eight input waveguides of the second combining cascade 32 are the eight output waveguides of the first combining cascade 32, and the sixteen input waveguides of the first combining cascade 32 are indexed relative to the eight output waveguides of the first combining cascade 32 in least significant inserted bit order.
Each of the sixteen output waveguides of branching region 16 connects to one of the sixteen input waveguides of combining region 20 via interconnection region 18. Which input waveguide of combining region 20 a particular output waveguide of branching region 16 connects to is determined by interchanging the first and second halves of the output waveguide's index, as shown in the following table:
output waveguide connects to input waveguide 0000 0000 0001 0100 0010 1000 0011 1100 0100 0001 0101 0101 0110 1001 0111 1101 1000 0010 1001 0110 1010 1010 1011 1110 1100 0011 1101 0111 1110 1011 1111 1111
For clarity, these connections are not shown explicitly in FIG. 2.
FIG. 7 is a schematic diagram of a 1.times.2 splitter 26 implemented as a directional coupler. An input waveguide 36 leads into a coupling waveguide 38, which in turn leads into an output waveguide 42. Coupling waveguide 38 is close and parallel to another coupling waveguide 40, which leads into another output waveguide 44. Coupling waveguides 38 and 40 both are of length L. Coupling waveguides 38 and 40 are covered by respective electrodes 46 and 48.
Coupling waveguides 38 and 40 are sufficiently close that the evanescent field of light propagating in coupling waveguide 38 overlaps with and is coupled into coupling waveguide 40. The strength of the coupling is characterized by a coupling coefficient .kappa., such that in a distance l=.pi./(2.kappa.), all of the optical energy entering waveguide 38 is transferred by this coupling to waveguide 40. The distance l is called the transfer length. The ratio of l to L is defined herein as the "normalized coupling length" of 1.times.2 splitter 26.
In one type of 1.times.2 directional coupler splitter 26, L is chosen to be equal to l, so that the normalized coupling length of this type of 1.times.2 splitter 26 is equal to 1. With no voltage applied to electrodes 46 and 48, this type of 1.times.2 directional coupler splitter 26 is in a "crossover" state, as described above, in which all of the optical energy entering directional coupler splitter 26 in input waveguide 36 is transferred to output waveguide 44 via coupling waveguide 40. To switch this type of directional coupler splitter 26 into a "straight-through" state, in which all of the optical energy entering directional coupler splitter 26 in input waveguide 36 leaves directional coupler splitter 26 via output waveguide 42, opposite voltages are applied to electrodes 46 and 48 to alter the refractive indices of coupling waveguides 38 and 40 sufficiently in opposite directions, thereby altering the coupling coefficient .kappa., so that the transfer length l of directional coupler splitter 26 becomes L/2, and all of the optical energy, that is transferred from coupling waveguide 38 to coupling waveguide 40 after propagating for a distance L/2, is transferred back to coupling waveguide 38 after propagating a distance L.
In another type of 1.times.2 directional coupler splitter 26, the normalized coupling length is equal to 1/2. With no voltages applied to electrodes 46 and 48, this type of 1.times.2 directional coupler splitter 26 is in an "all-pass" state: only half of the optical energy entering this type of 1.times.2 directional coupler splitter 26 via input waveguide 36 is transferred to output waveguide 44, and the remaining optical energy leaves this type of 1.times.2 directional coupler splitter 26 via output waveguide 42. This type of 1.times.2 directional coupler splitter 26 is placed in either the crossover state or the straight-through state by the application of appropriate voltages to electrodes 46 and 48.
FIG. 8 is a schematic diagram of a 1.times.2 splitter 26 implemented as a Mach-Zehnder interferometer. Input waveguide 36 is coupled, by a splitting mechanism 52, to an upper branch waveguide 54 and a lower branch waveguide 56. Splitting mechanism 52 may be a y-branch coupler, as drawn, or may be an active 1.times.2 splitter such as a directional coupler splitter. Upper branch waveguide 54 leads into a coupling waveguide 38', which in turn leads into output waveguide 42. Lower branch waveguide 56 leads into another coupling waveguide 40' that is close and parallel to coupling waveguide 38' and that leads into output waveguide 44. Coupling waveguides 38' and 40' both are of length L. Upper and lower branch waveguides 54 and 56 are partially covered by respective electrodes 58 and 60.
Like coupling waveguides 38 and 40 of FIG. 7, coupling waveguides 38' and 40' of FIG. 8 are sufficiently close that the evanescent field of light propagating in coupling waveguide 38' overlaps with and is coupled into coupling waveguide 40'. Here, too, the strength of the coupling is characterized by a coupling coefficient .kappa., such that in one transfer length l=.pi./(2.kappa.), all of the optical energy entering waveguide 38 is transferred by this coupling to waveguide 40.
As in the case of directional coupler splitter 26, a Mach-Zehnder splitter 26 may have a normalized coupling length of 1 or a normalized coupling length of 1/2. In the case of a Mach-Zehnder splitter 26 with a normalized coupling length of 1, with no current flowing through electrodes 58 and 60, all of the optical energy, that enters via input waveguide 36, exits via output waveguide 44. To cause the input optical energy to exit via output waveguide 42, sufficient current is applied to one of electrodes 58 or 60 to heat the respective branch waveguide 54 or 56 so that the resulting change in the refractive index of the respective waveguide 54 or 56 is sufficient to change the relative phases of the light in coupling waveguides 38' and 40' so that all the optical energy that enters via input waveguide 36 now exits via output waveguide 42 instead of output waveguide 44.
In the case of a Mach-Zehnder splitter 26 with a normalized coupling length of 1/2, with no current flowing in electrodes 58 and 60, half the optical energy entering this Mach-Zehnder splitter 26 via input waveguide 36 leaves this Mach-Zehnder splitter 26 via output waveguide 42, and the other half of the optical energy leaves this Mach-Zehnder splitter 26 via output waveguide 44. Application of the appropriate current to one of the electrodes 58 or 60 causes all the input optical energy to leave this Mach-Zehnder splitter via output waveguide 42, and application of the appropriate current to the other electrode 58 or 60 causes all the input optical energy to leave this Mach-Zehnder splitter via output waveguide 44.
Although in principle Mach-Zehnder splitter 26 need be fabricated with only one electrode, either electrode 58 or 60, to enable this switching, in practice both electrodes are fabricated because the fabrication process alters the properties of branching waveguides 54 and 56, and fabricating both electrodes 58 and 60, by inducing identical changes in the properties of branching waveguides 54 and 56, preserves the symmetry of Mach-Zehnder splitter 26.
Because of the symmetry of Mach-Zehnder splitter 26, it is arbitrary which of the two output states is designated as a crossover state and which is designated as a straight-through state. For consistency with the description of directional coupler splitter 26, the state in which all input optical energy emerges from Mach-Zehnder splitter 26 via output waveguide 42 is considered herein to be the straight-through state, and the state in which all input optical energy emerges from Mach-Zehnder splitter 26 via output waveguide 44 is considered herein to be the crossover state.
Note that the all-pass states of both directional coupler splitter 26 and Mach-Zehnder splitter 26, that have normalized coupling lengths of 1/2, are passive states. Directional coupler splitter 26 that has a normalized coupling length of 1/2 is in its all-pass state when no voltages are applied to electrodes 46 and 48. Mach-Zehnder splitter 26 that has a normalized coupling length of 1/2 is in its all-pass state when no current flows in electrodes 58 and 60.
By exchanging the roles of the input and output waveguides, 1.times.2 splitters 26 illustrated in FIGS. 7 and 8 are transformed into 2.times.1 combiners 28. In the straight-through state of such a 2.times.1 combiner 28, all of the optical energy input via input waveguide 42, and none of the optical energy input via input waveguide 44, emerges via output waveguide 36; and in the crossover state of such a 2.times.1 combiner 28, all of the optical energy input via input waveguide 44, and none of the optical energy input via input waveguide 42, emerges via output waveguide 36. In the all-pass state of such a 2.times.1 combiner, half of the energy of the optical signals entering via input waveguides 42 or 44 is superposed in output waveguide 36, with the other half of the energy being lost to scattering.
The numbers of input waveguides 12 and output waveguides 14 need not be powers of two, and need not be equal. FIG. 3 shows a classical 4.times.3 tree network architecture, for connecting four input waveguides 12 to three output waveguides 16. The waveguides of FIG. 3 are indexed as in FIG. 2. As in FIG. 2, the output waveguides of branching cascades 30 emerge from branching cascades 30 in least significant inserted bit order relative to the input waveguides of branching cascades 30, and the input waveguides of combining cascades 32 enter combining cascades 32 in least significant inserted bit order relative to the output waveguides of combining cascades 32. The following table shows how the output waveguides of branching region 16 are connected to the input waveguides of combining region 20 in interconnection region 18:
 output waveguide connects to input waveguide 0000 0000 0001 0100 0010 1000 0100 0001 0101 0101 0110 1001 1000 0010 1001 0110 1010 1010 1100 0011 1101 0111 1110 1011
Note that the optical switch networks of FIGS. 1-3 are reversible. Output waveguides 14 can be used as input waveguides, and input waveguides 12 can be used as output waveguides, with combiners 28 used as splitters and splitters 26 used as combiners. For example, the tree network architecture illustrated in FIG. 3 is also the architecture of a classical 3.times.4 tree network.
For the output waveguides of branching region 16 to connect to the input waveguides of combining region 20, these waveguides must cross each other extensively. Waveguides that cross each other must do so at a sufficiently large angle to preclude crosstalk. If the number of input waveguides 12 and output waveguides 14 is large, it is difficult to achieve this without making interconnection region 18 unreasonably large.
There is thus a widely recognized need for, and it would be highly advantageous to have, a tree network architecture with lower crosstalk and better space utilization than known tree network architectures.